Leonidas was one of three brothers: he had an older brother Dorieus and a younger brother Cleombrotus, who ruled as regent for a while on Leonidas' death before the regency was taken over by Pausanias, who was Cleombrotus' son.
Leonidas succeeded his half-brother Cleomenes I, probably in 489 or 488 BC, and was married to Cleomenes' daughter, Gorgo.
His name was raised to heroic status as a result of the events in the Battle of Thermopylae, one of the most famous battles in ancient history.

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Upon receiving a request from the confederated Greek forces to aid in defending Greece against the Persian invasion, Sparta consulted the Oracle at Delphi. The Oracle is said to have made the following prophecy in hexameter verse:[1]

Hear your fate, O dwellers in Sparta of the wide spaces;

Either your famed, great town must be sacked by Perseus' sons,

Or, if that be not, the whole land of Lacedaemon

Shall mourn the death of a king of the house of Heracles,

For not the strength of lions or of bulls shall hold him,

Strength against strength; for he has the power of Zeus,

And will not be checked till one of these two he has consumed.

In August 480 BC, Leonidas set out to meet Xerxes' army at Thermopylae, where he was joined by forces from other Greek city-states, who put themselves under his command to form an army between 4,000 and 7,000 strong. This force was assembled in an attempt to hold the pass of Thermopylae against a massive Persian army of between 80,000 and 290,000 men-at-arms who had invaded from the north of Greece under Xerxes I. Leonidas took only his personal bodyguards,[2] and not the army, because the majority of the Spartan Army was coordinating with the massed naval forces of the Greeks against the Persian Navy. This is contrary to the belief that the army could not be sent because of religious restrictions.

Xerxes waited 4 days to attack, hoping the Greeks would disperse. Finally, on the 5th day they attacked. Leonidas and his men repulsed the Persians' frontal attacks for the fifth and sixth days, killing roughly 20,000 of the enemy troops and losing about 2,500 of their own. The Persian elite unit known to the Greeks as "the Immortals" was held back, and two of Xerxes' brothers (Abrocomes and Hyperanthes) died in battle.[3] On the seventh day (August 11), a Malian Greek traitor named Ephialtes led the Persian general Hydarnes by a mountain track to the rear of the Greeks.[4] At that point Leonidas sent away all Greek troops and remained in the pass with his 300 Spartans, 900 Helots, and 700 Thespians who refused to leave.
Another 400 Thebans were kept with Leonidas as hostages.
The Thespians stayed entirely of their own will, declaring that they would not abandon Leonidas and his followers. Their leader was Demophilus, son of Diadromes, and as Herodotus writes: "Hence they lived with the Spartans and died with them."

One theory provided by Herodotus is that Leonidas sent away the remainder of his men because he cared about their safety. The King would have thought it wise to preserve those Greek troops for future battles against the Persians, but he knew that the Spartans could never abandon their post on the battlefield. The soldiers who stayed behind were to protect their escape against the Persian cavalry. Herodotus himself believed that Leonidas gave the order because he perceived the allies to be out of heart and unwilling to encounter the danger to which his own mind was made up. He therefore chose to dismiss all troops except the Thespians and Helots and save the glory for the Spartans.[5]

The small Greek force, attacked from both sides, was cut down to a man except for the Thebans, who surrendered. Leonidas was killed, but the Spartans retrieved his body and protected it until their final defeat. Herodotus says that Xerxes' orders were to have Leonidas' head cut off and put on a stake and his body crucified. This was considered sacrilegious.[6]

The tomb of Leonidas lies today in the northern part of the modern town of Sparta. Additionally, there is a modern monument at the site of the Battle of Thermopylae, called the "Leonidas Monument" in his honor. It features a bronze statue of Leonidas. A sign, under the statue, reads simply: "Μολών λαβέ" ("Come and get them!") which the Spartans said when the Persians asked them to put down their weapons.

"Leonidas Returns" is a movement of the Covenant suite from the game Halo 3

Leonidas is a chain of Belgian chocolate stores, with a Spartan helmet as its logo.

Leonidas became an Internet meme, with Gerard Butler's (see above) portrayal of him screaming "This Is Sparta!". This spawned a series of parodies, in which Leonidas' face is superimposed on someone else's, accompanied with the latter phrase.